A variety of different mechanical actuator systems are known for the purpose of moving mechanical bodies. These include, for example, fluid systems utilizing pneumatic and hydraulic cylinders, electric motor driven screw mechanisms, or other mechanical devices. Each type of actuator system has a set of associated characteristics and features, including accuracy, range of force or power it can apply, and cost. For example, fluid systems, such as pneumatic cylinders, while they exhibit fairly accurate control when they are operating against stops for controlling their position, are not very accurate at mid-stroke positions between such stops. Pneumatic actuating systems are able to apply large forces and high speed but are very sensitive to variations in load and to other working conditions, such as air pressure change and the stroke position due to the significant compressibility and high non-linear property of the pressurized air, and consequently may assume one position with one particular load and working condition and other positions for other loads and conditions for the same control input signal.
Other systems, such as a screw mechanism driven by an electric motor, exhibit very accurate control, or superior controllability, and flexibility, but are limited in the force or power they can apply to the load. The accuracy deficiencies of fluid systems are sometimes overcome by the use of servo hydraulic valves, but such valves are very complicated and very expensive and are sensitive to inaccuracies created by fluid contamination.
In high performance applications, the dc servo motor still predominates because of its versatility. However, in recent years there is a trend that ac servo motors are beginning to replace dc servo motors on many fronts where controllability is of primary importance. The advantages of an ac servo motor are robustness, overload capability and maintenance free feature. Nevertheless, the control system and algorithm of an ac servo motor are much more complex than that of a dc servo motor, therefore the cost of an ac servo system is high. Although electric motor systems have many superior control features, the limitation of power availability, small ratio of torque-to-weight-and-size size, and high cost as power output of the electric motor increases are their common disadvantages.
It is therefore an object and feature of the present invention to combine two different actuator and control systems in a manner which does not provide a mere compromise between the desirable force or power characteristics of one and the desirable accuracy characteristics of the other. Rather, they are combined in a manner which provides at least the same, close tolerance performance and accuracy of a more accurate actuator system with the desirable high power or force characteristics of the less accurate, higher power range system.
U.S. Pat. No. 4,782,258 discloses a robotic joint actuator, utilizing both the quick response time of an electric motor and the high torque capacity of a pneumatic motor by mechanically linking them together so that the electric motor and the fluid motor drive the same load. It discloses a controller, including a computer, which adjusts the fluid motor torque so that the electric motor torque is decreased to prevent the electric motor from overheating, and adjusts the electric motor torque to compensate for torque deficiencies of the fluid motor. That structure is directed to the transient torque characteristics of the two motors and does not attempt to control position or motion of a mechanical body because its control system is entirely different than that of the present invention.